In internal combustion engines, the exhaust aftertreatment of the combustion gases takes place by means of catalytic converters situated in the exhaust gas system of the internal combustion engine. However, since these catalytic converters have not yet reached the temperature necessary for converting pollutants, in particular after a cold start of the internal combustion engine, conversion of the pollutants in this cold start phase is inadequate. Electrically heatable catalytic converters are known from the prior art, in which the catalytic converter may be heated by an electric heating element, independently of the exhaust gas flow. The period of time in which the catalytic converter has not yet reached the temperature necessary for converting pollutants may thus be shortened. It is therefore possible to convert harmful exhaust gas components more quickly after a cold start, and thus to reduce the emissions of a motor vehicle.
The operation of an electrically heatable catalytic converter requires an actuator for the power supply. For example, a controlled power converter that has devices for current and voltage measurement may be used as an actuator.
In general, the component temperature of the catalytic converters in the exhaust gas system of a motor vehicle is of interest, on the one hand to monitor the functioning of the catalytic converter, and on the other hand to stay within temperature limits, above which the functioning of the catalytic converter is limited and/or there is risk of thermal damage to the catalytic converter.
In principle, the component temperature of a catalytic converter may be determined by using temperature sensors in the exhaust gas system. However, this results in a corresponding cost increase for the exhaust gas system. Therefore, the component temperature is generally determined based on models that calculate the component temperature of the catalytic converters in the exhaust gas system based on a measured or calculated exhaust gas temperature.
By use of an electrically heatable catalytic converter, it is possible to increase the temperature of the catalytic converter in the cold start phase, and thus to speed up the conversion of harmful exhaust gas components after the cold start. Such an electrically heatable catalytic converter may be situated in a position near the engine, or also in an underbody position of a motor vehicle. In determining the component temperature based on a calculation model, due to the numerous environmental influences, uncertainties are to be expected which increase with increasing distance of the catalytic converter from the exhaust outlet of the internal combustion engine. Therefore, for a catalytic converter in the underbody position of the motor vehicle, only inadequate modeling of the exhaust gas temperature may be possible, and the determined temperature may correspondingly differ greatly from the actual component temperature.
It is known from the prior art to determine the component temperature in an electrically heatable catalytic converter based on the resistance of the electric heating element. Since the heating resistance of an electrically heatable catalytic converter is generally highly dependent on the component temperature, and increases with increasing component temperature at the same current intensity, the component temperature of the electrically heatable catalytic converter may be derived from the heating resistance for a known voltage and a known current intensity.
A method for determining the component temperature of an electrically heatable catalytic converter is known from DE 89 05 073 U1, the catalytic converter being made up of electrically conductive trays, and the temperature being determined by a temperature-related change in the resistance.
It is known from DE 10 2004 046 917 to determine, in an electrically heatable catalytic converter, the temperature of the catalytic converter via the change in the resistance of the electric heating wire, wherein the electrical heating of the catalytic converter is switched off as soon as the component temperature determined in this way is above an activation temperature (light-off temperature) of the catalytic converter, and the catalytic converter is then further heated chemically by the catalytic reaction.
DE 10 2011 017 675 A1 discloses a method in which the temperature characteristic curve of an electrically heatable catalytic converter is determined based on the temperature-related change in the electrical resistance of a heating element of the electrically heatable catalytic converter.